For the improvement in yields of semiconductors, it has become important to immediately clarify the cause of occurrence of a defect in manufacturing process. At present, at the site of manufacturing semiconductors, defect analysis is performed by using a defect inspection apparatus and a defect observation apparatus.
The defect inspection apparatus is an apparatus that observes a wafer with optical means or electron beams and outputs detected defect coordinates. Since it is important for the defect inspection apparatus to process a wide range at high speed, the amount of image data is reduced by increasing the pixel size of an image to be obtained as much as possible (that is, by decreasing resolution). Therefore, in many cases, while the presence of a defect can be confirmed from the detected low-resolution image, the type of the defect cannot be determined in detail.
Thus, the defect observation apparatus is used. The defect observation apparatus is an apparatus that takes an image of defect coordinates of the wafer at high resolution based on the output from the defect inspection apparatus and outputs the image. With the advance of microfabrication in the semiconductor manufacturing process, the defect size has reached the order of several tens of nm, and a resolution power on the order of several nm has been required in order to observe the defect in detail.
For this reason, a defect observation apparatus using a scanning electron microscope (review SEM) has been widely used in recent years.
In a mass-production line of semiconductors, automation of observation operation is desired, and the review SEM has an ADR (Automatic Defect Review) function of automatically collecting the images at defect coordinates in a sample.
ADR is a function of automatically collecting the images of defect parts taken at a high magnification based on defect coordinates obtained from the defect inspection apparatus. The problem to be solved here is an error between defect coordinates output from the defect inspection apparatus and actual defect coordinates.
In general, since a variation of about ±4 [μm] is present as an error, when taking an image of the defect coordinates output from the defect inspection apparatus at a high magnification with a field of view of about 2.5 [μm] (for example, 50,000 times), there is a possibility that a defect may not be within the field of view.
For its solution, an image is first taken at a first magnification with a field of view of about 9 [μm] (for example, 15,000 times), a defect is next detected from the low-magnification image, and finally, an image of the detected defect is taken at a second magnification (for example 50,000 times).
Japanese Patent No. 3893825 (Patent Document 1) discloses, as a defect detecting method, a comparative inspection method in which a defect image obtained by taking an image of a defect part at a low magnification and a non-defective product image obtained by taking an image of a part where the same pattern as that of the defect part is formed at a low magnification are compared and a difference between these two images is detected as a defect.
Since a plurality of same chips are arranged on a semiconductor wafer, an image obtained by taking an image of a location moved by one chip from the coordinates where a defect is present can be used as a non-defective product image.
In recent years, with an increase in diameter of a semiconductor wafer, the number of defects to be reviewed per one wafer is increased, and the throughput of the review SEM is lower than that of the defect inspection apparatus. For these reasons, speed-up of ADR has been required.
In general, it takes a large amount of time for ADR to move a stage from an initial position to a destination position and take a non-defective product image and a defect image. A method of not only speeding up these processes but also omitting some of them in the image-taking procedure is effective for the speed-up of ADR.
Normally, the process in the procedure to be omitted is a process of taking a non-defective product image. For example, a method of previously preparing a non-defective product image and a method of synthesizing a non-defective product image from a defect image and performing a comparative inspection have been suggested.
As an example of the former method, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-67243 (Patent Document 2), there is a method in which a cyclic pattern is previously stored in a memory area or the like as a non-defective product image and a comparative inspection is performed by using this image and a defect image, thereby detecting a defect.
As an example of the latter method, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-98114 (Patent Document 3), there is a method in which local areas or the like with a similar appearance on a defect image are compared, a normal part is detected based on a probability distribution, a degree of reliability of a differential area with respect to defect detection is calculated, and the differential area with a high degree of reliability is detected as a defect. In addition, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-40910 (Patent Document 4), there is a method in which a repeated cycle of a circuit pattern taken in a defect image is used to synthesize a non-defective product image, and a defect is detected by a comparative inspection with the synthesized non-defective product image.